[SI-LIST] Re: characteristic impedance at DC
- From: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 10 Apr 2012 00:23:05 +0000
Sorry, my bitmap drawing didn't make it through
the reflector. The drawing had two boxes. The
box on the left side contained a Thevenin driver
and the box on the right side contained a question
mark.
Arpad
====================================================
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Muranyi, Arpad
Sent: Monday, April 09, 2012 7:03 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: characteristic impedance at DC
I have a problem with the statements that the impedance
is very low, on the order of a few milliOhms or so at
low frequencies or DC.
Consider this circuit:
[cid:image001.png@01CD1683.626893C0]
How would you measure the impedance in the box on the right?
You would measure it by measuring the voltage and current
that is in the wire connecting the two boxes, right? What
would you get if the box contained a capacitor and the source
on the left would be a DC source? You would measure the full
voltage of that source with 0 current, i.e. the impedance
would be infinite. Why would the T-line exhibit a few milli
Ohms, if the insulating material is not leaking significantly?
I think that idea comes from measuring the series resistance
of the T-line, from its left end to its right end. But that
is not the load impedance a driver (such as the box on the left
in the above diagram) would see...
Thanks,
Arpad
===============================================================
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Todd Westerhoff
Sent: Monday, April 09, 2012 5:54 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: characteristic impedance at DC
Mohammad,
Characteristic Impedance (sometimes called surge impedance) is the
impedance seen by a propagating wavefront. It's been covered at length
multiple times, including this overview in Wikipedia:
http://en.wikipedia.org/wiki/Characteristic_impedance
DC impedance from a typical transmission line ranges from nil to a few
ohms, depending on the line geometry and length. It's amenable to direct
physical measurement in many cases. Here's a device that can be used to
affordably make such measurements with 4 digits of precision:
http://www.radioshack.com/product/index.jsp?productId!03176
Hope that helps,
Todd.
Todd Westerhoff
VP, Software Products
Signal Integrity Software Inc. * www.sisoft.com<http://www.sisoft.com>
6 Clock Tower Place * Suite 250 * Maynard, MA 01754
(978) 461-0449 x24 * twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>
"I want to live like that "
-Sidewalk Prophets
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx]<mailto:[mailto:si-list-bounce@xxxxxxxxxxxxx]>
On Behalf Of mohammad haaeri
Sent: Monday, April 09, 2012 6:31 PM
To: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] characteristic impedance at DC
Hi,
What is the characteristic impedance of a transmission line at DC? If you
are saying Z0=sqrt(Rdc/Gdc) at DC, since Gdc=0, and Rdc is not zero,
therefore Z0 is infinite. Is it correct?
How does behavior of L, R, G, and C (line parameters) change vs. frequency
(at low and DC, and at very high frequency)?
Can Z0=sqrt(R+jwl/G+jwc) be used for all frequencies?
Thanks,
mohammad
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